The technique of material deposition by means of sputtering is known already for many decades, and hence need not be explained in detail here. It suffices to say that typically a plasma is generated in a low pressure chamber in which an inert gas such as Argon, or an active gas such as oxygen or nitrogen is present, and that a high voltage is applied between a so called “sputter target” (containing the material to be deposited) and a “substrate” upon which a layer of the sputter material is to be deposited. The Argon atoms are ionized, and the sputter target is bombarded by the Argon ions, so that atoms are freed from the sputter target, and move to the substrate, where they are deposited.
Basically three kinds of sputter targets are being used: planar circular disk targets, planar rectangular targets, and rotational cylindrical targets.
Typically three kinds of power source are being used: DC power, AC power (e.g. at a frequency of 1 to 100 kHz) and RF power (e.g. at a frequency of 0.3 to 100 MHz). DC power is typically used when the sputter target contains an electrically conductive sputter material. AC power is typically used when the deposited layer is less conductive. In the past, when a material with a low conductivity was to be sputtered, one typically added doping atoms to increase the conductivity of the target material.
However, the above-described techniques do not work anymore when an insulating material having a resistivity higher than for example 10 k ohm·cm is to be deposited without being allowed to add dopants, e.g. in applications where even a minor degree of contamination is detrimental for the performance or the lifetime of the deposited layer on the substrate, such as for example electrochromic windows, thin film batteries and solar cell applications, where even minor amounts of conductive particles can cause internal leakage or short-circuiting.
In US2013/0008777A1 Choquette et al. disclose an apparatus for depositing a.o. oxides by using RF power, without the need for metal doping, addressing the contamination issue. Choquette et al. propose a cylindrical rotating sputtering cathode (also referred to as “magnetron”) made of a non-magnetic material (such as stainless steel) within which a stationary tubular electrode is inserted, along with magnets attached thereto, as can be seen in FIG. 1 of said publication, replicated herein as FIG. 1. However, when using a sputter target (disk or rectangular or cylindrical) with relatively large dimensions (e.g. 2 to 4 meters, or more) in combination with RF frequencies (e.g. 1 to 100 MHz, or more), standing wave effects (SWE) start playing a role, resulting in a non-uniform electrical field, and hence also in a non-uniform deposited layer. While Choquette et al. address the issue of contamination (by employing RF power in combination with an un-doped sputter material) and the problem of heating (by using a cooling liquid and a non-magnetic material for the electrode and the shaft), the problem of “a standing wave effect” is not mentioned, nor addressed. Choquette seems to concentrate on mechanical issues, rather than electro-magnetic aspects.
US2014/0183037 describes an RF sputtering system 200 (replicated herein as FIG. 2) with a rotary target 201 wherein a single power source 202 and a power splitter 203 is used to apply two (same) RF power fractions at a frequency of 40 MHz to both ends of the metal backing tube of the target. It is alleged that by doing so a voltage node which would otherwise occur at about 1 m distance from the ends, can be removed, and a more uniform electrical field is created. This publication also describes another embodiment (not shown herein) whereby RF power is applied to a single end of the sputter target, while the other end is terminated by a variable terminating impedance. The impedance is varied over time so as to obtain a more uniform electrical field. While the general idea of providing RF power at both ends of a backing tube is described, no specific details are given about how this can be achieved. Hence, there is still room for alternatives and/or for improvement of this system.